The present invention relates to an electrophoretic liquid crystal display device.
Electrophoretic display devices have been known for some years. They have the benefit of bistability and high brightness, which are desirable for paper-like reflective-mode displays. The displays typically comprise a pair of opposed substrates provided with electrode patterns on their inner surfaces. Sandwiched between the substrates is a non-conductive liquid in which is dispersed highly scattering or absorbing microparticles. The microparticles become electrically charged, and can be reversibly attracted to one surface of the display by application of a suitable electrical field across the electrode structures. A problem with such displays is that they lack threshold, ie, the particles begin to move at a low voltage, and move faster as a higher voltage is applied. This makes the technology unsuitable for conventional multiplexed (matrix-addressed) displays, which require a relatively sharp threshold to reduce crosstalk.
It has been proposed in U.S. Pat. No. 4,305,807 to achieve a threshold by using a liquid crystal as the non-conductive liquid. The inner surfaces of both substrates are treated to give uniform planar alignment, in which the liquid crystal molecules lie substantially flat in the absence of an applied voltage with the director at the front surface being parallel to the director at the rear surface. When a voltage of sufficient magnitude is applied, the liquid crystal molecules switch from the planar alignment to a homeotropic alignment in which they align parallel to the electric field, perpendicular to the plane of the substrate surface. According to U.S. Pat. No. 4,305,807, the liquid crystal in the homeotropic state now presents relatively low hindrance to motion of the particles because the viscosity of the medium has dropped, permitting the particles to move to a cell wall. The threshold is therefore the threshold switching voltage for the liquid crystal. A problem that we have found with such a display is that the orientational effect can only be realised for small concentrations of particles—less than about 10%. As is known, for a sufficient optical effect, electrophoretic devices are usually doped with 25% or more of pigment particles. Consequently, such devices provide only a weak contrast. We have also found that planar treatment of the surfaces leads to strong sticking and aggregation of the pigment particles to the planar surfaces, so that the device cannot provide good parameters for practical applications.
Recently, a switching threshold has been reported in an electrophoretic device by R C Liang, Jack Hou and HongMei Zang, IDW '02 pp 1337-1340, and in WO 02/100155. The authors describe an active matrix electrophoretic display which has plastic substrates and is manufactured by roll-to-roll technology. The possibility of providing a switching threshold is mentioned and the threshold characteristics of such a device are given, but no explanation is given of how this is achieved. However, the same construction of the electrophoretic display is described by M A Hopper and V Novotny, IEEE Transactions on electron. devices, vol. ED-26 No. 8, 1979, pp 1148-1152. Here, the threshold effect is connected with a bonding between the pigment layers and the walls. It is shown that the threshold is quite poor and is not appropriate for a highly informative passive matrix display. Taking into account that the device in IDW '02 pp 1337-1340 has a similar construction and that the given threshold characteristic is not sharp enough, there will be some problems for designing a highly informative passive matrix electrophoretic display.
Also known are electrophoretic devices in which the particles move due to a lateral flow effect in a liquid crystal medium.
See, for example, EP 1 154 312. These devices need a complex, in-plane configuration of the electrodes and also the switching is quite slow. Another type of electrophoretic display is described in U.S. Pat. No. 6,441,881, in which a complex arrangement of slanted partitions is provided between the cell walls. The display changes either by movement of particles both from one surface to another and laterally, or by deformation of a liquid-crystal-filled microcapsule.